The Institute of Electrical and Electronics Engineers (IEEE) has adopted a set of standards for wireless local area networks (LANs), known as 802.11. Wireless products satisfying 802.11a, 802.11b, and 802.11g are currently on the market, for example.
Recently, an 802.11n standard, known also as the Enhancement for High Throughput wireless standard, has emerged. Under the 802.11n standard, transmitters and receivers each have multiple antennas for transmission and reception of data. As a multiple input, multiple output (MIMO) technology, 802.11n is designed to coordinate multiple simultaneous radio signals, and is expected to support a bandwidth of greater than 100 megabits per second (Mbps). In addition to MIMO, the 802.11n standard includes other features to increase throughput.
MIMO techniques supported by 802.11n include spatial multiplexing, beamforming, and channel training. Spatial multiplexing works by dividing the transmission, known as a frame, into different streams and sending them over multiple paths in the channel, using the multiple radios. At the receiver, the different streams are recombined to get the originally transmitted frame. Beamforming is a technique in which the transmitter and receiver use sounding to obtain an optimal transmission path. Multiple directional antennas are used to spatially shape the emitted transmission to beam the energy into the receiver over the optimal transmission path. Channel training, such as sounding, is a kind of measurement that allows a transmitter to know about the channel between the transmitter and the receiver. In a handshaking operation, the transmitter sends special frames to the receiver, which direct the receiver to measure or estimate the channel. The receiver sends the estimation data back to the transmitter, such that both the transmitter and receiver are trained as to the channel characteristics.
A wireless local-area network, or WLAN, consists of a number of nodes, known as access points (APs) and client stations (STA). The nodes communicate with one another using frames. Where the nodes operate under 802.11n, they may operate in one of three modes: legacy (802.11a, b, and g), mixed mode (802.11n and legacy), or Greenfield (802.11n only). When sending frames to another node, the node is a transmitter; when receiving frames from another node, the node is a receiver. Transmissions on the wireless network are random (pseudo-random) access.
Each node communicates with another node according to one or more data rates supported by the receiver. Each node may have its own supported data rate set. A subset of the supported data rates at which each node associated in the same basic service set (BSS) may transmit and receive frames is called the Basic Rate Set for that BSS.
In earlier versions of 802.11, transmissions occurred according to data rates specified in the basic rate set. In 802.11b, for example, the supported data rates are 1, 2, 5.5, and 11 Megabits per second (Mbps), in the 2.4 Gigahertz (GHz) frequency band. In 802.11a, the supported data rates are 6, 9, 12, 18, 24, 36, 48, and 54 Mbps, in the 5.2 GHz frequency band. For 802.11g, the supported data rates are 1, 2, 5.5, 6, 9, 11, 12, 18, 24, 36, 48, and 54 Mbps, in the 2.4 GHz frequency band. The basic rate set reflects these supported data rates. Devices supported by these standards may automatically adapt their data rates, based on channel conditions.
The different standards also are characterized by different modulation techniques. 802.11b devices use direct sequence spread spectrum (DSSS) while 802.11a devices employ orthogonal frequency division multiplexing (OFDM). 802.11g devices employ a combination of the 802.11a and 802.11b modulation techniques.
For 802.11n, the transmission rates are considerably more complex. A modulation coding scheme (MCS) is used to specify the transmission rate. MCS includes variables for the modulation scheme, the number of spatial streams, and the data rate on each stream. The number of spatial streams is based on the number of antennas, with up to four antennas supported. For nodes on the network to communicate with one another, a negotiation takes place between them to determine the optimum MCS based on the present channel conditions. The MCS is continuously adjusted as the channel conditions change. There exist 77 modulation coding schemes specified in the latest 802.11n draft (as of January, 2007), with eight of them being mandatory.
Under 802.11, when a transmitter sends a frame, the receiver sends an acknowledge frame, known as ACK frame. The frame transmitted may include data, and may be transmitted at a high rate. The response frame (ACK) may be transmitted at a much slower rate. Frames transmitted at a lower rate generally have better propagation, that is, the low rate frames travel farther, and thus may reach more nodes in the wireless network.
The 802.11 standard also defines a number of control frames used for different purposes. Control frames for collision avoidance, protection of sequences of data frames, acknowledgement, and polling, for example, are available under the new standard. Most of the control frames are sent at one of the rates specified in the basic rate set of the node transmitting the control frames.
The new features of the 802.11n standard extend the usage of the control frames for more purposes: requesting different types of feedback and responding with feedback. Examples of such a request/response are: modulation coding scheme (MCS) request and response, explicit feedback (EF) request and response, implicit feedback (IF) request and response, and so on.
The MCS request is sent so that the transmitter can obtain the optimum supported rate of the receiver. The criteria used to send the MCS request is different from the criteria used to send other control frames. The 802.11n standard employs a receiver assisted link adaptation protocol, in which the transmitter sends an MCS request, and the receiver measures the characteristics of the link between the transmitter and receiver. The receiver then sends an MCS response, which includes the recommended MSC to be used by the transmitter.
The control frames are used in the 802.11n standard for the same purposes as in the previous (legacy) versions of the 802.11 specification. However, the control frames are also used for new functions. In some cases, there is conflict between the control frame requirements of the legacy standard and the 802.11n standard.
As one example, the legacy approach for transmitting all the control frames except a block ACK request (BAR) and block ACK (BA) by one of the basic rates and the BAR/BA by supported rates cannot be used for the high-throughput standard. For example, a high-throughput (HT) station is associated with a HT access point (AP). The AP transmits data using transmit beamforming, but the HT station does not use transmit beamforming. If the AP sends a BAR by the rate it uses for data transmission and the station responds with BA by the same rate, following the existing rule, this response will not succeed.
As another example, if a transmitting station wants to determine an optimum MCS rate for communicating with a receiving station, the transmitting station sends an MCS feedback request to the receiving station, preferably using MIMO transmission for the request. Optimally, the MCS feedback obtained is used in a subsequent control frame transmitted by the transmitting station. The MCS request may be sent by a request-to-send (RTS) frame, and getting a response, together with a clear-to-send (CTS) frame, and using the responded MCS by transmission of the first data frame.